Military Mixes "Smart" Coatings

Convoys of United States Army tanks are rumbling across Kuwait this month-ready again to play a key role in war with Iraq. But before they fire a single shot, those tanks are already locked in battle with old foes: whipping desert sands, blistering sun, and even the air itself.

Each year, the axis of corrosion costs the U.S. Army $10 billion dollars-$2 billion for painting and scraping alone, labor-intensive work that’s hazardous to people and the environment. So last fall, the Army’s Tank-automotive and Armaments Command, Armament Research, Development and Engineering Center (TACOM-ARDEC) in Picatinny Arsenal, NJ asked a coalition of researchers at the New Jersey Institute of Technology, Clemson University, and the University of Illinois to do something about it. The command awarded them $838,000-and promised up to $1.5 million more-to find materials that combine self-healing characteristics with the ability to change color and sense structural damage or environmental changes. Dan Watts, the man leading NJIT’s program, says their research has been pushed along by unexpectedly rapid advances in self-healing polymers and electronics made from carbon nanotubes that go “beyond the realm of interesting academic speculation and approach economic practicality.”

Finding the Right Mix

For several years, materials researchers have pursued self-healing polymers that can be applied to a surface-such as industrial machinery-and that would resist the deterioration that occurs over time (see “Nano Biomaterials,” TR, November 2002). But no one has yet found a material that can repair itself repeatedly, work as a thin coating, be stored for years, and-for military applications-resist chemical agents. To that end, the Picatinny group sought out Nancy Sottos, a professor of mechanics at the University of Illinois at Urbana-Champaign. Sottos’s lab engineered an epoxy with micron-scale capsules that burst when cracks form, quickly sealing them. (Click for animation.) “The key to making it a product is the shelf life,” Sottos says. “Right now, we mix them and break them up right away.”

In addition to self-repairing properties, the Army’s ideal smart coating must also incorporate nanoscale devices that detect corrosion at it happens-perhaps, Watts says, by sensing movement in the material.